1. Field of the Invention
The invention relates to a control apparatus and control method for an internal combustion engine equipped with a turbocharger, and more specifically, to a control apparatus and a control method for the internal combustion engine based on a flow rate in a compressor of the turbocharger.
2. Description of the Related Art
As means for estimating a flow rate of the air taken into cylinders (hereinafter referred to as “cylinder intake-air flow rate”), a method using a physical model of an air intake system is available. Japanese Patent Application Publications No. 2005-155384, 2006-22764, and 2003-293821 (JP-A-2005-155384, JP-A-2006-22764, and JP-A-2003-293821) and Published Japanese Translation of PCT application No. 2003-518581 (JP-A-2003-518581) describe methods of estimating the cylinder intake-air flow rate of an internal combustion engine equipped with a turbocharger. In the internal combustion engine equipped with a turbocharger, a compressor model that models a compressor is used as one of the physical models of the air intake system. In the compressor model, a flow rate of the air flowing through the compressor (hereinafter referred to as the “compressor flow rate”) is calculated based on a plurality of parameters, such as pressures upstream and downstream of the compressor, and a turbine speed.
However, a conventional compressor model is designed focusing on the flow rate characteristics when the compressor is in a supercharging state. Therefore, when the pressure downstream of the compressor is higher than the pressure upstream thereof, the compressor flow rate can be estimated with high accuracy. However, due to the aforementioned reason, there is a problem that when the pressure downstream of the compressor is equal to or lower than the upstream pressure, the accuracy in estimating the compressor flow rate is degraded, and the compressor flow rate is estimated to be lower than the actual flow rate. The pressure downstream of the compressor becomes lower than the upstream pressure when, for example, a throttle is rapidly opened to rapidly accelerate a vehicle.
The cylinder intake-air flow rate is calculated based on the compressor flow rate. Therefore, if the compressor flow rate is estimated to be lower than the actual flow rate, the cylinder intake-air flow rate is also estimated to be lower than the actual flow rate. As a result, a fuel injection amount calculated based on the estimated cylinder intake-air flow rate results in becoming smaller than the required amount corresponding to the actual cylinder intake-air flow rate, and the air-fuel ratio is therefore shifted to the lean side.